arch-nspawn: setarch to CARCH

Allows calling makechrootpkg without worrying about the architecture
makechrootpkg: Ensure we have a writable PKGBUILD

For pkgver updates.
makechrootpkg: Split out chrootbuild into a function

Now syntax highlighting works properly! :D
arch-nspawn: Quiet systemd-nspawn again

systemd-nspawn always outputs some debug messages over stderr.
Both stdout and stderr from inside the chroot are sent through
a pty to stdout.
archbuild: Correct makechrootpkg argument order

The user-passed makechrootpkg_args may contain a "--" to pass
arguments to makepkg. In this case, the order is wrong.